Photographic systems of various types are well known, including silver halide, diazo and the like. However, silver halide photographic elements are relatively expensive and normally involve wet processing. Diazo systems have usually been considered slow in speed, are positive-working, and typically are processed using ammonia. Therefore, it has been desirable to provide a nonsilver photographic system which would be negative-working, would be comparable or superior to diazo in speed, and could be processed under dry conditions. One approach to such a system involves the photodegradation of a synthetic organic polymeric material.
For instance, Plambeck in U.S. Pat. No. 2,892,712, issued June 30, 1969, disclosed a process for preparing relief images by the photodegradation of various organic polymers, preferably those having a carbon to carbon polymer chain, but also including unstabilized formaldehyde polymer. Various photopolymerization catalysts or initiators were used. With preferred polymers, the process was not dry since a coating of a liquid solvent or swelling agent was present on the surface of the element during the exposure step. This was necessary because the polymer degradation process with preferred polymers tended to be reversible. In addition, development of the relief image was generally done by means of liquid developers. Furthermore, exposure times were very long because polymer degradation essentially ceased when the radiation source was removed; that is, the system was not capable of amplifying an exposure.
This invention concerns a radiation-sensitive element comprising a depolymerizable polymer, such as a polymer or copolymer of an aromatic 1,2-dialdehyde, and, optionally, a radiation-sensitive substance or combination of substances which upon absorption of radiation is capable of initiating the depolymerization reaction. The radiation-sensitive element may also contain a binder and/or a substance or combination of substances which is capable of forming a colored or fluorescent reaction product with the monomer produced in the depolymerization reaction.
This invention also comprises a process of exposing to radiation a composition comprising a depolymerizable polymer, such as a polymer of an aromatic 1,2-dialdehyde, and, optionally, a photosensitive substance or combination of substances which upon absorption of radiation gives a product capable of initiating the depolymerization reaction, for example, an acidic product. After exposure, the composition is heated to a temperature of up to about 150.degree.C, forming a monomer such as an aromatic 1,2-dialdehyde. A visible image can be formed during heating by the reaction of the released monomer with a color-forming substance or substances included in the element, by heating in contact with a separate element containing the color former, or by heat treatment in the presence of a gaseous color-forming reagent, such as ammonia. Alternatively, a fluorescent image may be produced by an appropriate choice of a substance or combination of substances which reacts with the monomer.
A positive-working radiation-sensitive process comprises exposing a radiation-sensitive element to radiation following which the element is heated or contacted with a solvent which removes the exposed areas but permits the unexposed areas to remain as a positive resist.
According to Barney U.S. Pat. No. 3,102,811, issued Sept. 3, 1963, it is known to employ the color forming property of a light-sensitive aromatic 1,2-dialdehyde in providing a copy process. The dialdehyde is exposed to a light source which destroys the aldehyde imagewise and the unexposed aldehyde is reacted with an ammonium salt to form a colored image. An exposure of up to 16 minutes might be required.
Poly(methyl methacrylate), (PMMA) has been used as a positive working resist material within the micro-electronic industry. The PMMA is typically exposed to an electron beam with an exposure charge density in the region of 5 .times. 10.sup.-.sup.5 to 5 .times. 10.sup.-.sup.4 coulomb / cm.sup.2 which causes random chain scission of the molecular chains. Development occurs by dissolving away molecules of low molecular weight but leaving unaffected those of high molecular weight. However, it has been desirable to have available a resist which is faster than PMMA.
The criteria on which a resist is evaluated for micro-electronic device fabrication are:
1. Speed PA1 2. Resolution PA1 3. Chemical Resistance PA1 4. Adhesion to substrate
It has been found both theoretically and in practice that higher resolution is obtainable by electron beam exposure than by optical methods. Electron beam sensitivity is therefore a practical requirement of the resist where a very high resolution is required.
The speed of positive-working electron-beam resists is expressed in terms of the incident charge required per unit area to render the coating completely removable. With PMMA, the minimum necessary exposure is 5 .times. 10.sup.-.sup.5 Coulomb/cm.sup.2. This is for a 0.4 to 0.8.mu.m coating, developed by a standard procedure in 3/1 (by volume) isopropanol/methyl isobutyl ketone as described by R. A Harris, J. Electrochem. Soc., 120, 270(1973).
It is an object of this invention to provide a photodepolymerization process of greater speed than that described above and which uses stable polymers. Another major object of this invention is to provide a process which is negative-working, to complement the existing positive-working diazo process. Further objects are to provide elements which may be processed under dry conditions, and which will give neutral images of greater stability than diazo images. A further object of this invention is to provide a process that can produce visible or fluorescent images of sufficiently high resolving power for micrographic applications, and that may also yield relief images for resist and chemical milling applications.